1. Field of the Invention
The present invention relates to a multi-optical axis photoelectric sensor configured by light projecting and receiving units each of which is formed by disposing a plurality of optical elements in a line.
2. Description of the Related Art
For example, a multi-optical axis photoelectric sensor is used in order to form a safety light curtain around a press machine. The multi-optical axis photoelectric sensor is configured by a light projecting unit and a light receiving unit each of which is formed by disposing a plurality of optical elements in a line. The light projecting unit and the light receiving unit are disposed opposite to each other so that light beams projected from respective light projecting elements of the light projecting unit are received by the corresponding light receiving elements of the light receiving unit, respectively. For example, when a worker shields a part of the light curtain, a light shielding signal is outputted from the multi-optical axis photoelectric sensor, whereby the press machine is immediately stopped during its operation.
A multi-optical axis photoelectric sensor disclosed in JP-A-2000-251595 is arranged in a manner that a circuit board is disposed at the rear surface of the optical elements, and a screw is inserted into a first hole formed in an optical block and also into a second hole formed in a casing corresponding to the first hole to thereby perform relative positioning between the casing and the optical block. Two sets each formed by the optical block and the circuit board disposed at the rear surface thereof are housed within the casing and these two circuit boards are electrically coupled by a flat cable.
A multi-optical axis photoelectric sensor disclosed in JP-A-2001-135208 is arranged in a manner that a board housed within a casing is formed by laminating a base board and a photoelectric element board. Further, a circuit to be coupled to the respective optical elements and on which the optical elements are disposed in a line is printed on the photoelectric element board. Resistors, capacitors and transistors are mounted on one surface of the base board which is opposite to the other surface facing on the photoelectric element board. In the photoelectric element board, two positioning holes are formed so as to be separated from each other. Further, two bosses capable of being inserted into these two positioning holes are formed in the optical block, whereby the bosses and the positioning holes perform the relative positioning between the optical block and the photoelectric element board. A first hole for positioning with respect to the casing is formed in the optical block and a second hole corresponding to the first hole is formed in the casing, whereby a screw is inserted into the first and second holes to thereby perform the relative positioning between the optical block and the casing.
According to the inventions disclosed in the aforesaid publications, the multi-optical axis photoelectric sensor having many optical axes can be formed by providing plural units each formed by the optical block and the photoelectric element board disposed at the rear surface thereof. However, a pitch between the end optical axis of one of the units and the end optical axis of another of the units varies depending on where the screw is fastened. It is important for the multi-optical axis photoelectric sensor to keep the distance (pitch) between the respective adjacent optical axes constant. In particular, when the pitch is set to be as short as possible in order to enhance the detection ability, it is important to keep the pitch constant.